Interpretive Summary: Grape growers with concerns about global climate change and the potential for warmer spring temperatures raised questions about the effects warmer spring temperatures on the date of budburst and growth in grapevines. We controlled the temperatures of dormant grapevine buds during about three weeks before budburst until a green leaf was seen emerging from the bud. Cooler and warmer temperatures were applied. Although the temperature of the buds was altered for only three weeks before shoots were growing, the effects were seen for the entire season: warmer buds produced shoots that grew faster and longer until fall, whereas cooler shoots began growing later, grew more slowly all season, and were shorter at the end of the season. Faster-growing shoots will require more water earlier in the season, which growers need to know for managing irrigation water. Shorter, slow-growing shoots may produce less sugar to mature the grape crop. Results provide information to growers both in cool climates and those with concerns about higher spring temperatures predicted by various climate models.

Technical Abstract:
Temporal and spatial variation in climate contributes to pronounced differences in shoot and canopy architecture within and between plants of woody perennials. We studied the influence of temperature near budbreak on shoot growth and architecture in grapevines (Vitis vinifera) while minimizing the effects of other microclimatic variables. Dormant buds and emerging shoots of field-grown grapevines were heated above or cooled below the temperature of ambient buds from before budbreak until individual flowers were visible on inflorescences, at which stage the shoots had four to eight unfolded leaves. Multiple treatments were imposed randomly on individual plants and replicated across plants. Shoot growth and development were monitored throughout two growing seasons. Higher bud temperatures advanced the date of budbreak and accelerated shoot growth and leaf area development. Differences were due to higher rates of shoot elongation, leaf appearance, leaf-area expansion, and axillary bud outgrowth. Although main shoots arising from heated buds grew most vigorously, apical dominance in these shoots apparently was reduced, as their axillary buds broke earlier and gave rise to more vigorous lateral shoots. Axillary bud outgrowth was minimal on shoots emerging from buds cooled below ambient. The variation in shoot development persisted or increased throughout the growing season, well after temperature treatments had terminated. Variation in early-season bud and shoot temperature may alter substantially grapevine shoot development. It appears that much of the variability in shoot architecture within and between plants may be established during the budbreak period and may be maintained or amplified during the growing season.